Thermometric apparatus



Aug. 2, 1949.

. Filed Oct. 19, 1944 DEFLECTION DEF-ECTION TIME M INVENTOR. g-nmam GIT- Patented Aug. 2, 1949 THERMOMETRIC APPARATUS Lawrence E. Smith, Hubbardston, Mass, assignor to The Bristol Company, Waterbury, Conn., a corporation of Connecticut Application October 19, 1944, serial No. 559,419v

This invention relates to fluid-filled ther- .mometers and more especially to a thermometer system of the type having a variable-volume element such as a Bourdon springconnected by means of a capillary tube to a, closed container or bulb, the whole enclosure so formed being filled with a volatile or expansible fluid. Thermometer systems of the above class, while sensitive, and possessing considerable force, whereby to position an'indicating, recording, or controlling member, are inherently subject to the time lag attendant upon the transfer of heat from the surface to the whole mass of the fluid within the bulb, and consequently are characterized by a response delay which may materially impair the merit of the instruments in which they are employed.

It is an object of the present invention to provide means whereby the characteristic over-all time of response in a thermometer of the fluidfille'd type may be materially lessened and the attainment of flnal readings correspondingly facilitated.

The invention also comprises the provision of mean-s of the above nature having an overshooting characteristic, which, while not interfering with the normal purposes of the instrument shall be advantageous in the actuation of contacts or other control couples in automatic regulation of temperature.

In implementing the purposes of the invention it is proposed to provide a thermometer comprising in combination two independent fluidfilled systems having diifering time-response characteristic-s, and different sensitivities, whereby, under the joint influence of said systems, the pointer, index, or control element to be positioned will attain its final reading or indication in a shorter time than would characterize the responseof a single system.

Other features of the invention will be hereinafter described and claimed.

In the" drawings:

Fig. 1 is a front elevation'ofan indicating thermometer embodyingthe principle of the invention.

Figs. 2 and-3 are graphic representations of the performance of thermometers embodying the principle of the invention,

Figs. 4 and 5 are representations of bulbs which may be utilized alternatively to the form shown in Fig. 1. r r Fig. 6 illustrates the application of the inventionrto automatic regulation of temperature by pneumatic. means.

12 Claims. (Cl. 236-79) Fig. '7 illustrates the application of the invention to a conventional form of electrical control.

In Fig. 1 is shown an indicating thermometer embodying the principle of the invention. Upon a base-plate III are mounted two Bourdon springs I I and I2 each adapted to deflect upon the application of internal pressure. Intermediate said springs ispositioned an index or pointer l3 adapted for limited angular deflection about a pivot mounting it upon the base plate ill. A measure of the degree of said deflection is provided by means of a graduated scale l5 fixed upon the base It), and juxtaposed to the index i3. An extended lever arm I6 is pivotally mounted intermediate its extremities upon the pointer l3, whereby to be angularly deflected about an axis parallel to, but displaced from that of the pivot mounting H and movable with said pointer. Said extremities are operatively connected' to the Bourdon springs ll and i2 by means of link members I! and. respectively, whereby said lever arm constitutes a differential lever, rendering the pointer subject to the joint influence of said Bourdonsprings and adapted by its angular deflection about the pivotal mounting II to provide a measure of the arithmetical difierence of their deflections. The arrangement, furthermore, is such that deflection of the Bourdon spring il in response to an increase of internal pressure will act through the link ill upon the diiferential lever i6, which, pivotally mounted about the point of attachment of the link it, will cause the index l3 to be deflected in a positive sense with respect to the graduation of the scale 85. Similarly, a deflection of the spring i2 in response to an increase ofinternal pressure will cause the index i3'to be deflected in a negative sense with respect to said scale. In the interests of simplicity of calibration and of proper scale proportionality it is desirable that the pivot point at.

which the link i8 is attached to the differential lever it'follow a path wherein at one position the axis of said pivot point may coincide with the axis of the stationary pivotal mounting it."

A bulb 20, connected by means of a capillary tu-be 2| to the Bourdon spring it provides therewith an enclosed space adapted to be filled with an expansible or a volatile fluid or combination of the same, whereby variations of the pressure of said fluid due-to changes in the temperature to which the bulb 2B is exposed will cause. the Bourdon spring. to-be' more or less deflected in response to-said changes. An enclosed'bulb 22,

which-for a liquid-filled system will be of considerably smaller internal volume than the bulb 2D, is similarly connected to the Bourdon spring l2 by means of a capillary tube 23, In the case of gas-filled or vapor-tension systems, the bulbs may be made of the same internal volume, and a difference in their sensitivities obtained by suit able difierence in the quantity, quality, or initial pressure of the filling fluid. In general, the bulb 20 is given a faster time response and a greater sensitivity than the bulb 22, (The term sensitivity" as herein used may be defined in accordance with one of its commonly accepted meanings as designating the degree of response of the deflecting element in relation to a given change of the measured magnitude.)

Since the efiects of similar pressure changes within the bulbs 20 and 22 upon the deflection of the pointer or index l3 are in opposite senses, it follows that the position attained by said pointer or index when a state of equilibrium exists will represent the diflerence in their calibrations. Adjustment is made such that temperature readings obtained with the bulb 20 alone in action will be greater than the true value of temperatures to which said bulb is exposed. The discrepancy between said readings and true temperature values is incorporated in the calibration of the smaller bulb 22, which tends to deduct from the deflection of the pointer an amount sufficient to provide a correct reading on the scale I5.

In operation, the bulbs 20 and 22 are placed in close thermal association so that they will both assume the temperature of the body or space upon which measurement is to be performed. While a steady condition exists, the joint influence of the two thermometer systems as imparted to the pointer or index l3 through the differential lever l6 and associated linkages will provide a true temperature reading upon the scale I5. Upon a change in temperature, the larger and more sensitive bulb 20 will be the first to respond, and will tend to cause the pointer or index I3 to approach its ultimate reading at a relatively rapid rate, to the extent that it may overshoot the calibrated point which corresponds to the actual temperature value. The bulb 22 and its associated system, while beginning to act at the same time as the system connected to the bulb 20, will lag behind the same, but will succeed in restoring the pointer or index Hi to the true temperature reading after an interval depending on the relative time response of the two bulb systems. The tendency, therefore, will be 'for the index l3 upon any change in measured temperature, to approach the true reading at a relatively rapid rate overshooting the same, and then being restored to the correct value by the action of the bulb 22 and its associated system.

The coordinated operation of the two bulbs having differing characteristics will be better understood by reference to the graphs shown in Figs. 2 and 3, wherein are plotted the deflections of the responding elements associated with the two bulbs over a lapse of time prior to and immediately following, the attainment of a steadystate condition. In each of these curves the zero of the vertical coordinates represents a reference temperature or steady-state value at which the bulb systems are assumed to be in equilibrium, and the ordinate value represented by the line T corresponds to the temperature to be measured, and to which the bulbs are simultaneously and instantaneously exposed. The curve A" represents the characteristic response of a single bulb having a system calibrated to read the actual temperature under measurement. It will be noted that the initial response may be characterized by a slight delay representing the time of transfer of the temperature change through the walls of the bulb, which of course provides no response. As heat from the source to be measured reaches the fluid within the bulb, expansion begins and the response follows a substantially exponential law whereby the exhibiting member asymptotically approaches a true indication of the temperature to which the bulb is exposed.

As indicated in Fig. 2, the response curve A" reaches of its final value in three time units,

and, if the curve were truly logarithmic, would attain 99% in six time units, and so on until the discrepancy between the indicated and the true values becomes negligible. As hereinbefore pointed out, the principal object of the present invention lies in reduction of time delay between a change of temperature to which the sensitive element is exposed and the ultimate indication of its value on the instrument scale.

The characteristic response of the Bourdon spring II in Fig. 1, in conjunction with the bulb 20, and without the modifying influence of the bulb 22, is as indicated in the curve B of Fig. 2 wherein the system, being of greater sensitivity than otherwise necessary, reaches an indication of the measured temperature in a relatively short time (two time units) and continues to increase according to the characteristic exponential law. The characteristic of the bulb 22 and associated system is made as indicated in the curve C of Fig. 2, wherein it will be noted that, while the ultimate sensitivity is much less, the time of response is made greater with respect to the ultimate value attained, than in the system represented by the curve B."

By suitably proportioning the respective characteristics of the two bulb systems, which are differentially connected as shown in Fig. 1, it is possible to obtain a characteristic, which, whilepartaking of the rapid response of the more sensitive bulb, will eliminate the excess of the reading derived from that bulb over and above the true value to be measured. As the curve (3" rises much less rapidly than the curve B in the time interval immediately succeeding an increase in temperature, there is little resultant effect on the response of the exhibiting member subject to the joint influence of the two systems. As the excursion of the exhibiting device approaches the true value of the temperature under measurement, the effect of the bulb system corresponding to the curve 0" becomes progressively more pronounced; and, by so proportioning and selecting this characteristic that the magnitude of deflection derived from the bulb 22 will be equivalent to the excess of that derived from the bulb 20 above the measured value, it becomes possible to have the index or pointer follow the curve D," wherein the ultimate reading is attained after the lapse of a relatively short time interval, and subsequently continues substantially unchanged so long as the temperature to which the bulbs are exposed remains constant.

In Fig. 3 is shown a group of curves similar to those in Fig. 2, the curve E," which represents the performance of the more-sensitive bulb, being identical with the curve B shown in Fig. 2. The curve F, representing the performance of the less sensitive bulb, acting in opposition to the more sensitive, is so selected that the response delay is even greater than that represented by the sensitive of the two associated bulbs may be delayed to any desired degree are well known in the art, and need here only be mentioned. The smaller bulb may be made with walls of relatively nation embodying the principle of the invention, in which there is a tendency for the initial excursion or the pointer or index to be exaggerated and subsequently .withdrawn as graphically shown in Fig. 3, may be utilized to considerable advantage in providing an anticipatory." characteristic'incertain classes of automatic temperature control. In Fig. 6 is shown an application of the principle to a pneumatic temperature great thickness, requiring a correspondingly long time for temperature changes external to the same to be communicated to the fluid withinthe bulb, or use may be made of a material having a thermal conductivity relative low as compared with that of the larger bulb. Also, a time-delay characteristic may be introduced by choosing for the connection between the small bulb and its associated Bourdon tube a capillary tubing having a bore small in comparison with that of the tube connected to the larger bulb. Further variations in sensitivities and time response characteristics can be efiected by the selection of filling fluids and their operating pressure ranges.

In Figs. 4 and 5 are shown bulb constructions in which the above-mentioned, and other principles are used more or less in combination for the purpose of obtaining the desired relative response rate between the two bulbs. In Fig. 4 is shown a combination in Which the functions of the main and auxiliary bulb units are incorporated in one integral assembly. A bulb member 25 is internally divided into a large chamber portion 26 and a small chamber portion 21, the latter having materially thicker walls than those of the portion 26. Communicating with the chamber portion 21 is a capillary tube 28; and communicating with the chamber portion 26 is a capillary tube 29. In use, the capillary'tubes 28 and 29 are connected to operating elements in a manner identical to the corresponding tubes 2| and 23 in Fig. 1, whereby there is provided a differential thermometer having all the character istics attributed to the form shown in Fig. 1. While the chamber portion 21 is shown as having thicker walls than the portion 26, the delayed response so obtained may alternatively be provided by forming the walls of chamberportion 21 of material having a lower thermal conductivity than that of the Walls of the portion 26. For example, the walls of the portion 26 may be formed of copper, and those of the portion 21 of iron.

'Again', the whole bulb 26 may be constructed of [such material as copper, and the smaller chamber portion 21 formed as an insert, providing either a thicker wall or a wall of material having a relatively low conductivity.

In Fig. 5 is shown an alternative form which may be assumed by the dual bulb constitutin the basis of the invention. The main bulb .comprises a closed chamber 36, and has in communication therewith a capillary tube 3| corresponding to the tube 2| shown in Fig. 1. Wholly enclosed within the chamber 36, and out of contact with its walls, is a smaller bulb 32 sealed from communication with the chamber 36, and having connected thereto a capillary tube 33, corresponding to the tube 23 in Fig. 1. The principle and performance of the form of bulb shown in Fig. 5 are substantially identical to those of that shown in Fig. 4, and to the combination of bulbs 26 and 22 shown in Fig. 1.

' The characteristic response of the bulb combicontrolling system of a conventional type. A tank or reservoir 46 adapted to contain a body of liquid whose temperature is to be maintained at a predetermined constant value, is provided with an inlet conduit 4|, through which maybe admitted steam or some other temperature-affecting agent from a source 42 under the control of a pneumatically actuated valve 43. A con trolling instrument 45 includes a pair of Bourdon springs or equivalent elements 46 and 41, corresponding in all respects to 'the' elements ii and i2 in Fig. 1, said springs each having one extremity fixed to the base of the instrument 45 and a free extremity adapted to deflect in response to variations in internal pressure. Said free extremities of said Bourdon springs are individually linked to the opposite ends of a diiferential lever 48 pivotally mounted upon an arm 49 adapted for deflection through a limited angle about a bearing adjacent to one of its extremities.

The interior of the Bourdon spring 46 is connected by means of a capillary tube 56 to a rela- 'tively large and rapidly responding bulb 5|,

exposed to the liquid in the tank 46. The interior of the Bourdon spring 41 is connected by means of capillary tube '52 to a small and slowly responding bulb 53 in intimate thermal association with the bulb 5|, and also exposed to the liquid whose temperature is to be regulated. The combination of Bourdon springs, bulbs and capillary tubes, together with the difierential lever 48 carried upon the defiectable arm 49 constitutes a thermometer having a response characteristic similar to that set forth in connection with Fig. l of the drawings.

Carried by the free extremity of the arm 49' is a vane element 55 cooperating with an orifice member 56 having escape jets, not shown in the drawing, whereby said vane by changes in its position with respect to said jets will react to vary the back pressure of air escaping therefrom. A sup-' ply of compressed air from a conduit 51 is led through a constriction 56 to a conduit 59 in communication with the orifice member 56 and also with a bellows member or capsular springfBIl operatively associated with a three-way valve'jfii of the supply-and-waste type. Compressedair from the conduit 51 is supplied to the valve6l, and the outlet of said valve is connected by means of a conduit 62 to the actuating element of the valve 43. The pneumatic control device as thus far described is the equivalent of that fully set forth in'U. S. Letters Patent No. 1,880,247 granted October 4, 1932 to H. L. Griggs and A. R. Mabey; and the functioning is such that changes in the position of the vane 55 with respect to the orifice 56, affecting the back pressure therein due to a greater or less obstruction of the escape of 1 air from the conduit 59 as admitted through the constriction 58, will produce corresponding variations in pressure within the bellows member 66, and thereby affect the setting of the valve 6| to modify the degree of opening of the valve 43.

Neglecting for the moment, the function or the Bourdon spring 41 and assuming the same to be stationary, it will be seen that, by suitable choice and proportionin oi the elements 01 the apparatus as thus far set forth, there may be eflected a regulation oi temperature within the tank-88 'as determinedby the bulb 8| and modified by the degree of admission of heating agent through the conduit ll. When the function 01' the Bourdon spring 41 is taken into consideration, the thermometer system associated with the bulb 8| and the Bourdon spring 86 being suitably calibrated, the action will be such that, according to the principles'hereinbefore set forth, the initial response of the vane 88 to any change in the temperature to be regulated will be first exaggerated, and subsequently withdrawn. This action will be reflected in the performance of the controller, wherein will be introduced a desirable anticipatory characteristic which has been found advantageous in meeting many problems of response delay in automatic control of temperature.

In Fig. 7 is diagrammatically shown an electrical circuit especially adapted to utilization of the principle of the invention for the purpose of automatic regulation of temperature. This circuit represents the well-known thermostatic" control, in which a solenoid-actuated valve is alternatively opened and closed according to the demand for a temperature-affecting agent, with no mid-point or position of equilibrium. Mounted upon a base plate 88 is a differential mechanism 88 in allrespects the equivalent of that shown in Fig. l, and having a defiectable pointer arm 61 subject to the joint influence of temperature changes in two thermally associated bulbs 88 and 89, in a manner identical to the action of the bulbs i and 58 upon the pointer 48 in Fig. 6. In this arrangement said pointer is given performance characteristics similar to the curve G in Fig. 3, first overshooting the true value by a small increment and subsequently being withdrawn to a position corresponding to the actual temperature.

Carried upon the arm 61 is a contact member 18 adapted for electrical engagement with either of two fixed contacts 1i-12, according to whether the temperature to which the bulbs 68 and 88 are exposed is below or above the desired value. Mounted upon the base 88 is a solenoid operated relay 18 having two sets of normally open contacts 18 and 18. both adapted to be 8 in the drawings, the contacts 1| and 12 are made adjustable, and are set to positions where, at the desired value or the regulated temperature, the contact 18 will be juxtaposed to, but in engagement with neither oi, said fixed contacts. It isin this relation that said contacts are shown in the drawings. the relay 13 and the valve 18 being deenergized, causing the latter to rest in its closed position.

It may now be assumed that the temperature to which the bulbs 68 and 88 are exposed is talling, whereupon the contact 10 will approach the contact 1 I. Neglecting for the moment the "anticipatory action of the device, it will be seen that as said last-named contacts are brought into engagement a circuit will be completed by the same from the line conductor 88 through the conductor 82 and the solenoid of the relay 18, together with a portion of the conductor 88 and the resistance 84, to the line conductor 8|, thereby energizin said solenoid in series with said resistor, and causing the contacts 14 and 18 to be closed. Closure of the contact 14 provides a path from the line conductor 88 through the conductor 85 and the solenoid of the valve 18 to the line conductor 8i, thereby energizing said solenoid and opening said valve to admit heating agent to the space whose temperature is to be regulated. Closure of the contacts 15 provides a connection between the conductor 88-82 thereby shunting the contact 10--1I, leaving the latter free to be separated by the motor of the arm 61 without interrupting the flow of current.

An increase of the temperature to be regulated, subsequent to, and consequent upon, the action initiated by engagement of the contacts 10-11 will thus produce no effect upon the control circuits until the arm 81 has deflected sufiiciently .far to bring contacts 10-12 together short-circuiting the solenoid of the relay 13 (current flow being limited to a safe value by the resistor 88) aud causing said relay to be deenergized, opening its contacts 14 and 15, these contacts functioning respectively to deenergize and close the valve 18, and to interrupt the flow of current through the contacts 10-12 and the resistor 84. Under these conditions the control system becomes inert and no further action can be initiated until the contacts 1P1! are again closed.

closed upon energisstion or the solenoid of said relay. A solenoid-actuated valve 18 is installed in a pipe or conduit 11 whereby to control the low of a heating agent through said conduit to an oven, tank, or other enclosed space (not shown in the drawing) wherein are located the bulbs 88 and 89. A source of electric power is represented by two conductors 88 and 8!. of which the former is connected to the contact 1| and also to one side of eachof the relay contacts 18, and 18 and the-latter is connected to one terminal of thesolenoid of the valve 18. The movable contact 18 carried by the arm .81 is. connected by means of a conductor 82, having a flexible portion to permit free movement of the arm 81, to one terminal of the actuating solenoid of the relay 18, and also to the free terminal of the contact 15. The contact 12 is connected by means of a conductor 88 tothe free terminal ofv the solenoid of the relay' 18, and also to'the lineconductor 8i through "a suitable current-limiting're'sistor 84. The free terminals of the relay contact 14 and the winding of the valve 18 are interconnected by means of a conductor 88.

By means well known in the art, and not shown 7 Bourdon tubes are of equal strength. variations possible by the differential combination of the bulbs 88-68, as hereinbefore set forth, is utilized, the performance of the electrically controlled valve 18 will be such as to anticipate, and therefore neutralize the characteristic delay of response. By properly selecting, proportioning and adjusting the component elements of the control system there may thus be provided a highly sensitive and-critical control characteristic in accord with the objects of the invention.

It willbe apparent that in a liquid-filled system constructed according to the principles of the invention, and so proportioned that the two in hydrostatic pressure dueto changes inelevation of the bulbs in relation to the instrument will tend to neutralize each other, with a corn sequent elimination of error due to such variations. It will further be obvious that changes in internal pressure due to ambient temperature of the Bourdon springs and capillaries will similarly be neutralized, with the resultant elimination of temperature error in the ultimate displacement of the index or pointer of the instrument.

The terms and expressions which I have employed are used as terms of description and not of limitation, and I have no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but recognize that various modifications are possible within the scope .of the invention claimed.

I claim:

1. In apparatus of the class described, the combination of two closed thermometer systems each comprising a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same, and each system containing an expansive fluid, said bulbs being adapted for simultaneous exposure to a temperature to be measured, a movable member, and means differentially connecting said responsive elements to said member for shifting the latter to positions corresponding to the difference in response of said elements, one of said systems having a response more rapid and of a magnitude greater than the rapidity and magnitude of response of the other system to the extent that the differential action of the two systems on said member will produce with a minimum of time lag a setting of said member representative of the actual temperature. I

2. In apparatus of the class described, the combination of two closed thermometer systems each comprising a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same; and each system containing an expansive fluid, said systems having different sensitivities and different time-response characteristics and said bulbs being adapted for simultaneous exposure to a temperature to be measured, a scale, a pointer movable relatively to said scale, and means difierentially connecting said responsive elements to said pointer for shifting the latter to positions along said scale corresponding to said temperature. 1

3. In apparatus of the class described, the combination of two closed thermometer systems each comprising a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same, and each system containing an expansive fluid, said bulbs being adapted for simultaneous exposure to a temperature to be measured, a scale, a pointer movable relatively to said scale, and means differentially connecting said responsive elements to said pointer for shifting the latter to positions along said scale correspondpansive fluid, said systems having different sensitivities and different time-response characteristics, and said bulbs being adapted forv simul taneous exposure to said temperature, a movable member, means differentially connecting said responsive elements to said member for shifting the latter to positions corresponding to the dif 'ference in response of said elements, and means controlled by said member for regulating the supply of a medium for affecting the temperature of said space.

5. Apparatus for regulating the temperature of a space, comprising two closed thermometer systems each having a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same, each system containing an expansive fluid, said systems having different sensitivities and different time-response characteristics, and said bulbs being adapted for simultaneous exposure to said temperature, a movable member, means differentially connecting said responsive elements to said member for shifting the latter to positions corresponding to the difference in response of said elements, and pneumatic means controlled by said member for regulating the supply of a'medium for affecting the temperature of said space.

6. Apparatus for regulating the temperature of a' space, comprising two closed thermometer systems each having a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same, each system containing an expansive fluid, said systems having different sensitivities and different time-response characteristics and said bulbs being adapted for simultaneous exposure to said temperature, a movable member, means differentially connecting said responsive elements to said member for shifting the latter to positions corresponding to the difference in response of said elements, and electrical means controlled by said member for regulating the supply of a medium for affecting the temperature of said space. i

'7. Apparatus for regulating the temperature of a space, comprising two closed thermometer systems each having a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same, said systems having different sensitivities and different time-response characteristies, and each system containing an expansive ing to the temperature to which said bulbs are exposed, one of said systems having a response more rapid and of a magnitude greater than the rapidity and magnitude of response of the other system to the extent that the differential action of the two systems on the pointer will produce with a minimum of time lag an indication of the actual temperature.

4. Apparatus for regulating thetemperature of a space, comprising two closed thermometer systems each having a bulb, a fluid-pressure-responsive element, and a tube operatively intercom necting the same, each system containing an exfluid, said bulbs being adapted for simultaneous exposure to a temperature to be measured, a movable member, means differentially connecting said responsive elements to said member, one of said 5 thermometer, systems being adapted upon change in said temperature to produce a response both rapid and of a magnitude. greater than that of said change and to impart said response to a portion of said differential means, and the other of said thermometer systems being adapted upon a change in said temperature to respond to said change more slowly and to act upon said differential mechanism first to permit overshooting of said movable member under the influence of said first system and then to nullify said overshooting, and means controlled by said member for regulating the supply of a medium for affecting the temperature of said space.

8. Apparatus for regulating the temperature of a space, comprising two closed thermometer systems each having a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same, said systems having different sensitivities and different time-response characteristics, and each system containing an expansive lac-mass exposure to a temperature to be measured, a movable member, means differentially connecting said responsive elements to said member, one of said Y thermometer systems being adapted upon change in said temperature to produce a-response both rapid and of a magnitude greater than that of said change and to impart said response to a portion of said differential means, and the other of said thermometer systems being adapted upon a change in said temperature to respond to said change more slowly and to act upon said differential mechanism flrst to permit overshooting of said movable member under the influence of said first system and then to nullify said overshooting, and pneumatic means controlled by said member for regulating the supply of a medium for affecting the temperature of said space.

9. Apparatus for regulating the temperature of a space, comprising two closed thermometer systems each having a bulb. a fluid-pressure-responsive element, and a tube operatively interconnecting the same, said systems having different' sensitivities and diiferent time-response characteristics, and each system containing an expansive fluid, said bulbs being adapted for simultaneous exposure to a temperature to be measured, a movable member, means differentially connecting said responsive elements to said member, one of said thermometer systems being adapted upon change in said temperature to produce a response both rapid and of a magnitude greater than that of said change and to impart said response to a portion of said diiferential means, and the other of said thermometer systems being adapted upon a change in said temperature to respond to said change more slowly and to act upon said differential mechanism first to permit overshooting of said movable member, under the influence of said first system and then to nullify said overshooting, and electrical means controlled by said member for regulating the supply of a medium for affecting the temperature of said space.

10. In a thermometer system, the combination of two bulbs of diifering characteristics as to both time-response and sensitivity, both adapted to attain a common temperature to be measured, and each containing an expansible fluid, two pressure-sensitive deformable elements, independent and mutually adjacent capillary tubes placing said bulbs individually and operatively in communication with said elements, and temperature-regulating means responsive tothe joint influence of the deformations of said elements.

11. In a thermometer system, the combination of two bulbs of differing characteristics as to both 12 time-responsive and sensitivity. both adapted to attain a common temperature to be measured, and each containing an expansible fluid, two pressure-sensitive deformable elements, independent and mutually adjacent capillary tubesplacing said bulbs individually and operatively in communication with said elements, temperatureregulating means, and means differentially responsive to the joint influence of the deformations of said elements for controlling said regulating means.

12. In apparatus of of the class described, the

combination of two closed thermometer systems each comprising a bulb, a fluid-pressure-responsive element, and a tube operatively interconnecting the same, and each system containing an expansive fluid, said bulbs being adapted for simultaneous exposure to a temperature to be measured, a movable member, means cooperating with said movable member in the production of an effect corresponding to a change in said temperature, and means differentially connecting said responsive elements to said member for shifting the latter to positions corresponding to the difference in response of said elements, one of said systems having a response more rapid and of a magnitude greater than the rapidity and magnitude of response of the other system to the extent that the differential action of the two systems on said member will produce with a minimum of time lag a setting of said member corresponding to the actual temperature.

LAWRENCE E. SMITH.

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